An end of a piece of optical fiber can be shaped into a cone or tapered section. The tip of the cone or tapered section forms a small optical aperture for transmitting light. This can be used as an optical probe in a "near-field" configuration in which light is coupled between two elements separated by a spacing less than a wavelength of the light. In the near-field configuration, the "far-field" approximation to the propagation behaviors of electromagnetic waves is no longer valid. One result of such near-field optical configuration is a spatial resolution higher than what is permissible by the far-field diffraction limit in optical sensing.
Near-field scanning optical microscopy ("NSOM") explores the above property of the near-field optical sensing to achieve a spatial resolution of down to about one fiftieth of a wavelength. This compares favorably to the far-field diffraction limit which is approximately one half of a wavelength.
A near-field scanning optical microscope can be implemented with an optical fiber probe. As with almost all scanned probe techniques, the performance of a near-field scanning optical microscope is significantly determined by the quality of the optical probe. One challenge in achieving such a high resolution is construction of a probe with an aperture of the size of the desired feature size.
FIG. 1 shows a typical structure of a fiber probe 100 which includes a fiber body 106, a cone or tapered section 104, and a tip aperture with a tip apex 102. To resolve a feature much smaller than one wavelength, the size of the tip aperture should be about the feature size. Several other parameters may also affect the performance of a fiber probe, such as transmission wavelength range, mechanical properties of the probe material, and the geometry of the tapered section 104 of the probe.
For a given fiber material, the quality of the probe is largely determined by the tip aperture and the geometry of the tapered section. In general, the tip aperture should be as small as possible since it determines the minimum resolvable feature size. One parameter for characterizing the size of the tip aperture is the radius of curvature of the tip apex 102. A smaller radius of the tip curvature can resolve smaller features and produces higher resolution.
Light coupling efficiency or light throughput is another measure of the performance of a fiber probe. The surface quality of the tip apex 102 and the geometry of the tapered section 104 can affect the light coupling efficiency. In particular, the length of the tapered section 104 should be small in order to increase the light coupling efficiency of the probe. This is at least in part due to the cross-section of the tapered section 104 decreasing from the fiber body 106 to the tip apex 102 to a diameter smaller than the wavelength. A longer tapered region 104 requires radiation to propagate a longer distance within a confined dimension smaller than its wavelength. This reduces the light energy transmitted through the region. Thus, a short taper section 104 and a large cone angle .THETA. are desirable.
One method of making fiber probes is mechanical pulling. A fiber is first heated by a laser beam or a filament to a soft state at an elevated temperature and subsequently is pulled to form a tapered section and a tip. The tapered section of a probe may be coated with metal except for an aperture at the tip apex.
Another method of making fiber probes is by chemically etching the fiber material by using an active chemical etchant solution. Chemical etching is advantageous over the mechanical pulling in that higher light coupling efficiency can be achieved. Zeisel et al. has shown that chemically etched optical fiber probes have light throughput of about 100-1000 times greater than mechanically pulled fiber probes. Zeisel et al., Applied Physics Letters, Vol. 68(18), p. 2491 (1996). It is recognized that chemically etched fiber probes usually have shorter tapered region, smoother tip surface, and smaller radius of tip apex curvature than mechanically pulled probes.
One etching approach involves full immersion of a mechanically cleaved fiber end into a hydrofluoric acid solution. This method usually produces a sharp tip and a long tapered section (e.g. 10 mm long for a silica fiber of 125 .mu.m in diameter). However, such a probe can be mechanically fragile. See, for example, Radojewski et al., International Journal of Electronics, Vol. 76(5), pp. 973-980 (1994).
An alternative etching approach uses a layer of protection liquid on top of the etchant liquid to automatically terminate the etching process. See, for example, U.S. Pat. No. 4,469,554 to Turner, and Hoffmann et al., "Comparison of mechanically drawn and protection layer chemically etched optic fiber tips", Ultramicroscopy 61, pp. 165-170 (1995). According to this method, a portion of a fiber is immersed in the etchant liquid. The tip formation takes place at the interface of the etching liquid and the protection liquid layer. The etching process is self-terminated as the fiber portion immersed in the etching liquid is etched away to form a tip within the protection layer and the etching liquid breaks away from the fiber tip.
Many NSOM instruments operate in the optical spectral range from about 375 nm to about 850 nm. Fiber probes are usually made of quartz glass (SiO.sub.2) fibers. A near-field scanning infrared microscope ("NSIM"), operating in the infrared region from submicron to about 10 .mu.m, is an advantageous extrapolation of the NSOM technique for several reasons. For example, not all molecules have absorption bands in the visible region but almost every molecule has absorption bands in the IR region. Therefore, NSIM instruments can be used to detect more molecular species than NSOM. Also, IR absorption bands provide direct information about the presence and nature of chemical bonds in the observed spectral region. Hence, NSIM can be a useful tool in many applications.
One suitable set of compounds for IR-transmitting fibers is the chalcogenides. Chalcogenide fibers have good chemical stability and are less brittle than the other families of compounds for IR fibers. Like optical fibers, they are also glasses and can be heat-pulled with a capillary puller in a fashion similar to pulling regular optical fibers. However, heat-pulling of chalcogenide fibers is much more difficult than heat-pulling of glass fibers (e.g., SiO.sub.2). Moreover, the throughput of heat-pulled fibers is low. For example, Hong et al. report low light throughput in a range of about 10.sup.4 to about 10.sup.6 in heat-pulled IR fibers (Proceedings of SPIE 2863, pp. 54, 1996). This probably is in part due to a long tapered section of several mm as shown in a micrograph of FIG. 2 which was produced by using a scanning electron microscope.